The infrastructure of cooling apparatuses comprises an external infrastructure supporting the shell of the cooling apparatus, i.e, its outside walls and the roof which it may possess, and an internal framework supporting the lining of the cooling apparatus.
Numerous atmospheric cooling apparatuses have a wooden infrastructure, which may either be the external infrastructure, or the internal framework, or the system comprising the external infrastructure and the internal framework.
These wooden infrastructures are formed from wooden structures. These wooden structures essentially comprise posts (columns) and beams intersecting at junctions. In order to ensure the stability of this system of posts and beams, and in particular to ensure its resistance to winds and earth movements up to a certain limit, cross bracings are added to this system. These cross bracings obliquely connect the junctions. They are diagonal elements which increase the resistance of the structure to horizontal stresses, and together with the posts and beams they constitute the structural elements of the structure.
In general, posts and beams intersect at junctions and are continued on both sides of these junctions. Together with other posts and beams situated in the same approximate vertical plane, they form a grid. The structure comprises braces situated within this grid between two diagonally adjacent junctions, and by gussets which connect the braces to each other beyond the junctions and which connect them to the posts and beams.
The gussets are connecting plates which transmit the horizontal loads of the beams (forces, tensions, stresses) to the braces and to the posts. These loads essentially originate from the stresses due to the wind and due to earth movements, and from the loadings on the adjacent braces.
Hitherto, simple flat metal plates, which are relatively thick and which are drilled with a plurality of holes for the passage of bolts, have always been used as gussets. These metal plates are fixed to the braces by means of bolts, with the interposition of washers. These washers are flanged or serrated washers (claw washers) and penetrate the wood with their claws or flanges. They transmit the forces between the wood and the bolts by shear and compression. The bolts transmit the forces between the washers and the gussets by shear. When serrated washers are used, the claws, which end in a sharp point, are driven into the wood by force, for example by nailing or in a press. When flanged washers (shear washers) are used, a circular groove of the dimensions of the flange of the washer is previously cut in the wood and these washers are then embedded in the wood.
All the wooden components are cut, drilled, countersunk, etc. in a workshop specialising in woodwork, whilst the assembly is erected on site, where all the assembly hardware (gussets, washers, bolts, nuts, etc.) is delivered. These metal parts, which are subjected to stress, which are in a hot humid atmosphere and which are constantly wetted by water containing various dissolved salts, are made of stainless steel or of another relatively noble metal and are therefore expensive parts, particularly the bolts and washers, which are machined parts. When assembly is effected using serrated washers, embedding these washers in the wood is an operation which is costly on site.
The forces to which the braces are subjected are not the same for all the braces; they depend on the location of the brace in the structure. Moreover the load on a brace is greater the closer it is situated to the ground. Consequently, if one washer is sufficient to transmit the force from a brace to a gusset at the upper level of the tower, two of them are required, for example, at the intermediate level, and three are required at the lower level, or even more (for example two, four and six, respectively).
As a result, the assembly hardware will comprise various gussets of different lengths, depending on the number of holes which they must have in order to receive the requisite number of bolts supporting the special flanged or serrated washers.
In practice, when the claws or the flanges of the washers transmit forces to the wood by subjecting it to compression, of the order of 100 kg/cm.sup.2, these forces are transmitted to the bulk of the brace in order to be propagated along the length of the latter, in practice by shear, of the order of 10 kg/cm.sup.2 and is thus ten times less.
The differences in length between the various gussets are relatively great. They are sufficiently great that there are cogent economic reasons for not making all gussets to a standard length equal to the greatest length necessary.